A common goal of filtration processes is to reduce the concentration of low molecular weight compounds in a fluid by permeating a portion of the fluid through a semi-permeable membrane. Said membrane has pores that are large enough to permit the permeation of the low molecular weight compounds but small enough to preclude the permeation of the high molecular weight compounds included in said fluid. Most commonly, the said permeation process is carried out by the application of a pressure to the fluid being processed to induce permeation of a portion of the solvent and the low molecular weight compounds.
It is common to carry out these types of processes by adding a suitable diluent, free of the target contaminants, to fluid being processed at the same rate as the rate of permeation. By this means the concentration of the high molecular weight compounds is substantially constant during the course of the process. In general the rate of permeation in a filtration process is controlled by the concentration of the high molecular weight compounds. In order to minimize the amount of membrane area needed to achieve a specified decontamination factor the process is carried out an optimum concentration of the high molecular weight compounds. This process of filtration accompanied by the addition of a diluent is known as diafiltration, and the diluent is called diafiltrate. Diafiltration is commonly used in the purification of biomolecules to reduce low molecular weight contaminants as well as to replace the buffer used to dissolve the target biomolecule.
There are two common methods of carrying out diafiltration processes. These are described in various monographs, for example, “Ultrafiltration and Microfiltration,” Munir Cheryan (1998) pp. 3012-3018. The batch processes uses a fixed volume of fluid contained in vessel. The fluid is circulated from the vessel through a membrane module and back to vessel, while adding diluent to the vessel to replace the fluid permeated. When the rate of addition of diafiltrate is the same as the rate of permeation the process is commonly referred to as “constant volume diafiltration.” A standard material balance on the low molecular weight compounds yields the following prediction for the decontamination factor, DFB.
                              DF          B                =                  e                                    (                              1                -                R                            )                        ⁢                                          V                                  d                  ⁢                                                                          ⁢                  f                                                            V                ret                                                                        (        1        )            In this equation Vdf is the total volume of diluent or diafiltrate used to achieve the specified decontamination factor and Vret is the (constant) volume of solution processed.
The second common process uses a series of interconnected stages. The fluid being purified passes from stage to stage. In each stage diafiltrate is added at a rate equal to the permeation in that stage. The decontamination factor, DFC, for this process is given by the following equation.
                              DF          c                =                              (                          1              +                                                (                                      1                    -                    R                                    )                                ⁢                                                      V                                          df                      ·                      S                                                                            V                                          ret                      ·                      S                                                                                            )                    2                                    (        2        )            Where Vdf·S and Vret·S are the volumes of diluent and retentate that have passed through a stage in the course of the process. A comparison of the two processes carried out at equal values of the retentate volumes yields the following equation for the ratio of diafiltrate volumes at equal decontamination factors, DF.
                                          V                          df              ·              S                                            V                          df              ·              B                                      =                  N          ⁢                                    ⌊                                                                    (                    DF                    )                                                        1                    N                                                  -                1                            ⌋                                      ln              ⁡                              (                DF                )                                                                        (        3        )            
Equation (3) shows that the staged process generally requires more diafiltrate, which means that it requires more membrane area to process a given volume of material in a fixed period of time. Only with a large number of stages is the amount of additional diafiltrate not significantly more than that required for a batch diafiltration process. However, the resulting system would be expensive on account of the increased number of stages and the complexity and large number of interconnections between stages.
In summary, constant volume diafiltration processes are used when buffer consumption and the size of the membrane modules required to perform the process need to be minimized, whereas staged processes are used where there is a need for short residence times and the lack of space for large batch (recirculation) tanks. Batch diafiltration processes are typically used in biopharmaceutical processing to reduce buffer consumption and minimize the amount of membrane area required.
U.S. Pat. No. 7,384,549, mentioned above, discloses single-pass TFF modules with and without internal diafiltration distribution, suitable for single-pass concentration and single-pass diafiltration processes, respectively (hereafter called “single-pass concentration modules” and “single-pass diafiltration modules”). Single-pass diafiltration modules offer the benefits of staged diafiltration systems as described above (low residence time of the fluid being processed and no recirculation loop), but without the complexity of a staged system. However, single-pass diafiltration modules are still more complex than single-pass concentration modules, and for that reason are not yet commercially available. Therefore initial applications of single-pass TFF technology have been limited to concentration processes and have not included diafiltration processes.